One of the major advances in olefin polymerization technology has been the development of commercially useful metallocene based catalyst systems. Among other advantages, metallocene catalysts allow the production of polyolefins with unique properties such as narrow molecular weight distribution. These properties in turn result in improved structural performance in products made with the polymers such as greater impact strength and clarity in films.
While metallocene catalysts have yielded polymers with improved characteristics, they have presented new challenges when used in traditional polymerization systems. One such area has been in the control of “sheeting” and the related phenomena “drooling” when metallocene catalysts are used in fluidized bed reactors such as those described in U.S. Pat. Nos. 5,436,304 and 5,405,922. By “sheeting” is meant the adherence of fused catalyst and resin particles to the walls of the reactor. The sheets will eventually dislodge from the wall and, if the sheets are large enough, they can result in reactor plugging. “Drooling” or dome sheeting occurs when sheets of molten polymer form on the reactor walls, usually in the expanded section or “dome” of the reactor, and flow along the walls of the reactor and accumulate at the base of the reactor. This can result in plugging of the distributor plate in the reactor and loss of fluidization.
In commercial reactors, sheets can vary widely in size, and are usually about 0.6 to 1.3 cm thick and are from 0.3 to 2.0 meters long, with a few even longer. They can have a width of about 7 cm to more than 45 cm. The sheets have a core composed of fused polymer which is oriented in the long direction of the sheets, and their surfaces are covered with granular resin which is fused to the core. The edges of the sheets can have a hairy or stringy appearance from strands of fused polymer.
It has been found that there exists a strong correlation between polymer sheeting and drooling and the presence of an excess of static charges, either positive or negative, in the reactor during polymerization. This is evidenced by sudden changes in static levels followed closely by deviation in temperature at the reactor wall. These temperature deviations are either high or low. Low temperatures indicate particle adhesion to the reactor causing an insulating effect from the bed temperature. High deviations indicate reaction taking place in zones of limited heat transfer. Following this, disruption in fluidization patterns is generally evident, such as, for example, catalyst feed interruption, plugging of the product discharge system, and the occurrence of fused agglomerates (sheets) in the product.
Various methods for controlling sheeting have been developed. These often involve monitoring the static charges near the reactor wall in regions where sheeting is known to develop and introducing a static control agent into the reactor when the static levels fall outside a predetermined range. For example, U.S. Pat. Nos. 4,803,251 and 5,391,657 disclose the use of various chemical additives to a fluidized bed reactor to control static charges in the reactor. A positive charge generating additive is used if the static charge is negative, and a negative charge generating additive is used if the static charge is positive. The static charge in the reactor is measured at or near the reactor wall at or below the site where sheet formation usually occurs, using static voltage indicators such as voltage probes or electrodes.
The prior art, such as that disclosed in U.S. Pat. Nos. 4,803,251 and 5,391,657, teaches that static plays an important role in the sheeting process with Ziegler-Natta catalysts. We have found that static also plays an important role in sheeting and drooling with metallocene catalyst. When the static charge levels on the catalyst and resin particles exceed certain critical levels, the particles become attached by electrostatic forces to the grounded metal walls of the reactor. If allowed to reside long enough on the wall under a reactive environment, excess temperatures can result in particle fusion and melting, thus producing the sheets or drools.
The principal cause for static charge generation in the reactor is frictional contact of dissimilar materials by a physical process known as the triboelectric effect. In the gas phase, polymer production reactors, the static is generated by frictional contact between the catalyst and polymer particles and the reactor walls. The frictional contact causes a flow of electrical charges from the walls of the grounded metal reactor to or from the polymer and catalyst particles in the fluid bed. The charge flow can be measured by employing static probes. Typical charge flows (currents) are of magnitude 0.1 to 10 microamperes per square meter of reactor surface area. Although these currents are very low, relatively high levels of electrical charge can accumulate over time in the reactor. This accumulation is enabled by the highly insulating characteristics of the polymer and catalyst particles.
The frictional electrification of the polymer and catalyst particles can be strongly influenced by the type of polymer that is being produced. In particular, the polymer molecular weight has a strong effect, with higher molecular weight polymers being more prone to developing high levels of static charge. Static charging in the fluid bed is also strongly influenced by the presence of minute quantities of static charge inducing impurities.
When sufficiently high levels of charge or charge accumulation becomes large enough, the frictional electrification of the polymer and catalyst particles can be strongly influenced by the type of polymer that is being produced. In particular, the polymer molecular weight has a strong effect, with higher molecular weight polymers being more prone to developing high levels of static charge. Static charging in the fluid bed is also strongly influenced by the presence of minute quantities of static charge inducing impurities.
For conventional catalyst systems such as traditional Ziegeler-Natta catalysts or Chromium-based catalysts, sheet formation usually occurs in the lower part of the fluidized bed. For this reason, the voltage indicators have traditionally been placed in the lower part on the reactor. For example, in U.S. Pat. No. 5,391,657, the voltage indicator was placed near the reactor distributor plate. See also U.S. Pat. No. 4,855,370. The indicators were also placed close to the reactor wall, normally less than 2 cm from the wall.
There are two types of static indicators (or probes) described in the prior art, the “voltage probe” (U.S. Pat. No. 4,855,370) and the “current probe” (U.S. Pat. No. 5,648,581 and U.S. Pat. No. 6,008,662). Both types of probes are similar in that they measure electrical characteristics of the fluidized bed near the reactor wall. The current probe measures the electrical current flowing from a metal electrode (probe tip) by the frictional contact of the resin and catalyst particles. It is intended to provide a single-point measurement of the surface current flowing from the much larger metal walls of the reactor to the fluid bed.
The voltage probe consists of a simple metal electrode connected to an external voltage measuring device of high resistance. Typical values of the resistance are of the order of 100 giga-ohms (1011 ohms). The authors of U.S. Pat. No. 4,855,370 mistakenly considered the readings from these probes to be an indication of the voltage within the fluid bed, as generated by the static charge. A more recent patent (U.S. Pat. No. 6,008,662) teaches that, despite the high resistance, the voltage probes actually measure the surface current. That is, the “voltage” indicated on the probes of U.S. Pat. No. 4,855,370 is actually just the product of the surface current times the resistance. Both types of probes are therefore functionally equivalent. They both measure surface current. As indicated above, typical values of surface current are on the order of 1 to 10 microamperes per square meter of reactor surface area.
It has been found that for metallocene catalyst systems, the use of a traditional voltage indicator has been ineffective in predicting the static charge in the fluidized bed and thereby reduces their effectiveness in preventing sheeting and/or drooling. This was completely unexpected based on the inventor's experience with olefin polymerizations in fluidized bed reactors and the teachings of the prior art. It is believed that this is due to the presence of a large amount of particle fines in the reactor. These fines accumulate at or near the reactor walls and hence near the static probes typically used in a fluidized bed reactor. These fines appear to prevent the polymer particles in the fluidized bed from transferring their charge to the static probes.
A proposed solution to the problem was to measure the static charge in the fluidized bed itself. To accomplish this, a needle probe was extended from the reactor wall into the heart of the fluidized bed. This probe failed to properly measure the static charge in the bed and sheeting occurred. The fines which coated the prior art probes also coated the needle probe, rendering it ineffective.
Thus it is apparent that a new method for determining that static charge in a fluidized bed reactor is needed, especially for use with metallocene catalyst systems.